Improve algorithm to find clusters

[SevereOverfl0w]

The problem with the current algorithm is that it presumes that all clusters have children nodes, this works by recursively finding all the parents of clusters until they "run out" (return nil).

As this is potentially expensive in large graphs, I re-use the result in subsequent recursions.

[SevereOverfl0w]

I'm a little unsure about whether I can replace the cluster? code, as I couldn't infer it's purpose.